Elsevier

Polyhedron

Volume 52, 22 March 2013, Pages 333-338
Polyhedron

Coordination mode-tuned stacking motif in alkali metal salts of Ni(pdt)2 complexes (pdt = 2,3-pyrazinedithiol) and its physical properties

Dedicated to Alfred Werner on the 100th Anniversary of his Nobel Prize in Chemistry in 1913.
https://doi.org/10.1016/j.poly.2012.09.014Get rights and content

Abstract

A series of alkali metal salts of bis(2,3-pyrazineditiolate) nickel complexes were synthesized. In these complexes, the Ni(pdt)2 units were arranged in one-dimensional stacks. The stacking motifs and resultant physical properties were quite different depending on the alkali metal cations, which are coordinated by the N atoms of the pdt ligands. Lithium salts showed alternating stacks of monomeric and dimeric Ni(pdt)2 moieties. Potassium and cesium salts showed one-dimensional stacks of monomeric and dimeric Ni(pdt)2 moieties, respectively. Sodium salt showed phase transition between monomeric and dimeric structures at around 220 K. The difference in physical properties was reasonably explained by the crystal structures.

Graphical abstract

A series of alkali metal salts of bis(2,3-pyrazineditiolate) nickel complexes, Ni(pdt)2, were synthesized. The stacking motifs of Ni(pdt)2 and resultant physical properties were quite different depending on the alkali metal cations. The difference in physical properties was reasonably explained by the crystal structures.

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Introduction

Sulfur- or selenium-containing redox-active π-conjugated molecules, such as tetrathiafulvalene (TTF) or Ni(dmit)2 (dmit = bis-1,3-dithiole-2-dithone) derivatives, have interesting conduction and magnetic properties. Their charge-transfer materials have been extensively studied from the viewpoint of not only basic science but also for applications in molecular devices. To date, a lot of interesting properties have been reported, such as superconductivity [1], conductive ferromagnet [2], spin liquid phenomena [3], single-component molecular metal behavior [4], etc. The strong conduction originates from the large overlap of the molecular orbitals through intermolecular interactions, such as π–π or S–S interactions.

Recent attention has focused on controlling the arrangement of such molecules and resultant physical properties via non-covalent chemical bonds, such as hydrogen bonds and coordination bonds. Rovira and co-workers have recently reported a new dithiolate metal complex, M(pdt)2 (M = Ni, Cu; pdt = 2,3-pyrazinedithiol) [5], and Mori and co-workers have reported pyrazine-fused TTF derivatives (TB-TTF = bis(pyrazino)tetrathiafulvalene [6], and pyra-TTF = pyrazinotetrathiafulvalene) [7]. These are π-conjugated redox-active molecules, which can be used as conductive or magnetic molecular units as well as ligands through the nitrogen atoms of the pyrazine moieties. Therefore, it should be possible to control the molecular arrangement by using supramolecular chemistry or crystal engineering. Using this strategy, we have reported two coordination polymers, Cu[Cu(pdt)2] and Na[Ni(pdt)2]·2H2O. Cu[Cu(pdt)2] is a metal–organic framework with N–Cu coordination bonds and shows a relatively high electrical conductivity (σ  10−3 S cm−1 at RT) [8]. Na[Ni(pdt)2]·2H2O is composed of one-dimensional stacks of Ni(pdt)2 moieties and shows a paramagnetic–diamagnetic phase transition accompanied by dimerization–dissociation of a Ni(pdt)2 moiety [9]. In Na[Ni(pdt)2]·2H2O, particularly, the stacking motif of the Ni(pdt)2 moieties strongly affects the magnetic and conductive properties, which can be tuned by changing the alkali metal ions. Herein we report the structure and physical properties of a series of alkali metal salts of Ni(pdt)2 complexes, Li2[Ni(pdt)2]3·8H2O (1), Na[Ni(pdt)2]·2H2O (2), K[Ni(pdt)2]·2H2O (3) and Cs[Ni(pdt)2]·H2O (4), which were tuned by the alkali metal ions used.

Section snippets

Materials

Ni(ClO4)2·6H2O, LiOH·H2O, NaOH and KOH were purchased from Wako chemicals. CsOH·H2O was purchased from Aldrich. Those were used without further purification. H2pdt was prepared according to literature method [5].

Synthesis of Li2[Ni(pdt)2]3·8H2O (1)

To a suspension of H2pdt (144 mg, 1.00 mmol) in methanol (10 ml) was added LiOH·H2O (84 mg, 2.0 mmol), and the solution was stirred until it became clear. An aqueous solution (90 ml) of Ni(ClO4)2·6H2O (183 mg, 0.500 mmol) was added and stirred for 30 min to obtain a stock solution of 5 mM Na2

Results and discussion

Crystallographic data of 14 are summarized in Table 1. Although we also synthesized rubidium salt of Ni(pdt)2 complex as a black needle shaped crystal, we have not succeeded in crystal structure determination.

Conclusions

In summary, we synthesized a series of nickel(bis-dithiolate) complexes, 14. The Ni(pdt)2 units in these complexes are arranged in infinite 1D chains parallel to another chains made by alkali metal–oxygen(water) coordination bonds as well as hydrogen bonds between water molecules. Those chains are connected by nitrogen(pdt)–alkali metal coordination bonds. The magnitude of the antiferromagnetic interactions for both complexes are different because of the difference in stacking motifs of the

Acknowledgments

This work was partly supported by a Grant-in-Aid for Creative Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. M.Y. thanks the financial support from Asahi Glass Foundation and Mistubishi Foundation.

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